Answer
$-\frac{y^2}{2}$
Work Step by Step
$36y^3=\underline6\times\underline6\times\underline y\times y\times y$
$72y=2\times\underline6\times\underline6\times\underline y$
GCF=$6\times6\times y=36y$
$-\frac{36y^3}{72y}=-\frac{36y^3\div36y}{72y\div36y}=-\frac{y^2}{2}$
